Method of metal electroplating



Patented Dec. 14, 1943 METHOD OF METAL ELECTROPLATING Henry A. HoldenPray, Columbus, Ohio, asslgnor to Battelle Memorial Institute, Columbus,Ohio, a corporation of Ohio No Drawing. Application July 10, 1939,Serial No. 283,580

1 Claim.

This invention relates to a method of metal electroplating, and moreparticularly to a method of producing a bright metallic plate over anelectrobuifed metallic surface.

When an article consisting of metals or alloys such as nickel, copper,or stainless steel, or an article having an electrodeposited orotherwise formed superficial layer of such metals or alloys, is made theanode in a suitable type of electrolytic bath and a current of properdensity is passed through the bath, a highly polished, lustrous metallicsurface can be obtained. Considerable difficulty, however, ha beenexperienced in obtaining clear metallic electrodeposits on suchclectrobuffed surfaces. Ordinarily, unless the electrobuifed metallicurface is first subjected to a mechanical buiilng operation, any platedeposited thereon will have a milky cast, which is very undesirable.This applies particularly to chromium deposits on elcctrobuffed nickelsurfaces. The cause for this difilculty is probably not a layer ofnickel oxide formed as the result of the anodic process, for cathodicreduction treatments are not remedial. A more likely manner of causationfor this trouble i outlined here inbelow.

Cold working, for example, by mechanical bufiing, of a metallic surfaceeffects a degradation of the surface crystals so that the immediatelysuperficial layer is either non-crystalline or of extremely fine grainsize, forming a so-called Beilby layer. An electrolytically buffedsurface, on the other hand, although highly polished and lustrous. ischaracterized by the absence of this Beilby layer and comprises crystalfaces of the same size asthose of the underlying metal. When chromium orother metals are electrodeposited on such electrobuffed surfaces, theresulting deposit either continues the grain structure of theelectrobuifed nickel or other metal or alloy or else deposition iseffected selectively upon the relatively large exposed surface grainsthereof. Either condition results in a chromium or other plate ofundesirable color.

Regardless of what may be the correct explanation of the milky castexhibited by metallic'plates deposited over electrobuifed metallicurfaces, I

v have now found that the milky appearance of such plates may beentirely obviated and a clear plate of chromium or other metal obtainedby following the electrobufilng operation' with a flash metalelectrodeposit applied directly to the electrobuffed surface. Whenchromium or other metals are then plated in the usual manner over theflash metal coating, the resulting plating has a clear color which inthe case of chromium is superior to the best heretofore obtainablechromium plate. The metal used in the flash coating is, forelectrobuffed nickel surfaces, preferably nickel itself, deposited fromany conventional or usual nickel plating bath operated under conditionsthat will give a flne grained deposit.

' It is therefore an important object of this invention to provide amethod of improving the color characteristics of metallic plates appliedover electrobuffed metallic surfaces.

A further and more specific object of the present invention is toprovide a satisfactory method of chromium plating over an electrobuifednickel surface.

Another important, object of this invention is to provide a method ofproducing a bright, clear colored metallic plate, when plating over anelectrobuifed metallic surface, the method including the step ofapplying a flash coating over the electrobufifed surface and thenplating directly upon the flash coating.

Other and further important objects of this invention will becomeapparent from the follow- .ing description and appended claims.

The present invention may be broadly defined as involving the use of asufliciently fine-grained deposit of any kind intermediate anelectrobuffed metal and a subsequent metallic electrodeposit, to preventcrystal continuity between the electrobufied metal and the finalmetallic deposit.

The method of my invention is particularly applicable to the chromiumplating of an anodically electrobuffed nickel surface, such as nickel oran alloy composed largely of nickel, or an electrodeposit of nickel. Theelectrobuffed surfaces in question, of course, do not include bright ormechanically buffed nickel surfaces, since. such nickel'surfaces neednot be electrobuffed. Various satisfactory electrolytic baths andmethods of anodic treatment to effect a highly polished, lustroussurface on nickel are well known, and, since they form no part of thepresent invention, re quire no further description. As explained above,however, if chromium is electroplated directly over an electrobuffednickel surface, the resulting chromium plate has an undesirable milkycast, the elimination of which is an object of my invention.

According to my present process, an electrobuffed nickel surface, priorto chromium plating, is given a flash coat of nickel or other suitablemetal, such as cobalt, nickel and cobalt, cadmium,

copper, or the like. By the term flash coating or plate is meant a veryrapid plating operation that results in a very thin electrodeposit. Asindicative of the thickness of a flash metal plate, but withoutlimitative effect upon the scope of the term, the thickness willordinarily be in the neighborhood of ti of an inch, or less.

Where a flash nickle plate is employed, the nickel may be deposited fromany suitable nickel plating bath in a manner well known to those skilledin the art. For instance, the flash nickel plate may be applied from aso-called bright nickel bath, or from an ordinary gray nickel bath, suchas the Watts bath, modified to operate at a low pH. As typical of thecompositions of these two types of bath, the following are given:

Bright nickel bath 240 g./l NiSO4L6HzO 45 g./l NiCl2.6H2O 3O g./1 H33036.75 g./l Naphthalene-1,5 di-sulfonic acid 1.5 g./1 ZnSO4.7H2O pI-I 2.5

With the above bath, using a current density of about 40 amperes per sq.ft. and a, temperature of 140 F., the time required for effecting asatisfactory flash plate is not more than from 30 seconds to 3 minutes,with 30 seconds generally sufficient.

Ordinary or gray nickel plating bath 262 g./l NiSO4.6H2O 6O g./lNiC12.6H2O 10.5 g./1 H3303 pH... 1.0

The above plating bath is a modified Watts type of bath, having a ratherlow pH and optionally containing sodium lauryl sulfate or other wettingagents. The length of time required to effect a flash plate is of thesame order of magnitude as that required when using the bright nickelplating bath. Excellent results have been obtained with the modifiedWatts type bath by operating the bath at a temperature of 130 F.- *10 F.and by using to 30 amperes per sq. ft. for 30 secs. to 3 min. with airagitation of the bath or simple oscillating movement of the work.

After the electrobuifed nickel surface has been coated with a flash orstrike plate of nickel, the

alysis, containing in the neighborhood of 250 g./l. of chromlc acid(CrOa) and 2 g./1. of sulfuric acid. The temperature and cathode currentdensity should be regulated within the well known limits required toobtain a mirror-like chromium plate. A bright, clear colored chromiumplate, without any milky appearance, is directly obtained when followingthe process of this invention.

One of the advantages of applying a flash plate of nickel, or othermetal, over the electrobuffed nickel surface, rather than resorting toother chemical or to mechanical means for eliminating the milky cast ofthe chromium plate, is that the flash plating operation is relativelyinexpensive and rapid. Even where ordinary gray nickel, as contrastedwith so-called bright nickel, constitutes the flash plate, it is so thinand fine-grained as not to have any noticeable effect on the brightnessof the after applied chromium plate. This is noteworthy, since ordinarygray nickel when deposited in the usual thickness employed as anundercoat from a chromium electroplate, must first be buffed, eitherelectrolytically or mechanically, if the subsequently applied chromiumplate is to have a mirror-like finish. Thus, the advantages ofelectrobuflin can be realized by following the process of my invention,without the difliculties heretofore experienced with respect to themilky cast imparted to the after applied chromium plate.

It will, of course, be understood that various details of the processmay be varied through a wide range Without departing from the principlesof this invention and it is, therefore, not the purpose to limit thepatent granted hereon otherwise than necessitated by the scope of theappended claim.

I claim as my invention:

In the method of metal electroplating wherein a nickel surface otherthan bright or mechanically buifed nickel is first electrobuifed andthen chromium plated, the improvement whereby a clear chromium plate isobtained, which comprises electrodepositing on the electrobuffed nickelsurface a flash coat of fine-grain nickel sufficiently thin to retainthe characteristically lustrous surface produced by electrobufiing, andthen chromium plating directly upon said thin nickel flash coat.

HENRY A. HOLDEN PRAY.

